Editors
Cave Story Editors
Booster's Lab
by Noxid
A Java-based editor and currently the most popular, due to its variety of features and cross-platform nature. It comes with a library of ASM hacks and an in-program function to apply ASM/hex patches and can be used to mod both freeware and CSE2/D-RS/etc as well as CS+.
Booster's Lab v0.5.1.1 (English)
Booster's Lab v0.5.1.1 packaged with JRE (English)
Booster's Lab v0.3.3.0 (Japanese)
Forum Thread Github Cave Editor
by Wistil
No longer updated and missing some features compared to Booster's Lab such as the ability to apply ASM/hex patches or edit CS+/CSE2/D-RS/etc, but its UI is more accessible. It is cross-compatible with Booster's Lab, meaning you can edit the same mod using both programs.
Cave Editor v0.99d
Source Code
Forum Thread Sky Dragon Editor
by Jakzie
The most recent editor. Still in development, but stable enough to create mods with, and contains a variety of QoL features such as the ability to export maps as an image and a more CE-like UI.
Sky Dragon Editor environment (for downloading the latest version)
Sky Dragon Editor v0.0.14
Github Page The King's Table
by Brayconn
Not recommended for beginners, but if you know what you're doing it can be used to mod most of the official console ports. The Avalonia (new) version was never finished, so it is best to stick with the WinForms (old) version for now.
WinForms version (more features)
Avalonia version (faster ui, less features)A more in-depth comparison table can be found here if needs be.